The space telecommunication systems, sometimes referred to Satcom systems, often use polarization as a supplemental degree of freedom to increase the spectrum efficiency in multi-beam frequency reuse scheme, and often use circularly polarized electromagnetic (EM) waves to avoid the problems associated with polarization misalignment. This approach is valid for both on-board satellite and terminal antennas. The generation of this circular polarization is known as a sensitive issue and is usually performed at feed level for a reflector antenna.
Most of the current on-board antennas for communication satellite applications, typically broadcasting and broadband applications operating at Ku and Ka band, usually produce circular polarization at elementary feed level by using a polarizing waveguide component, such as a septum polarizer or an iris polarizer. These polarizers are connected to the feeds, and a reflector antenna producing a multiple beam coverage will use as many polarisers as used feeds. These polarizers add mass and contribute to the bulkiness of the feed array, especially in low frequency bands, such as at L, S, C bands.
As an alternative sometimes implemented on terminal antennas of the user ground segment, low power elementary feeds in combination with a polarizing screen are used. This approach often requires a multi-layer design of the screens, resulting in relatively high insertion losses performance and increased manufacturing complexity. Such multi-layer screens are also characterized by relatively poor axial ratio performances over the scanning range and over the frequency bandwidth.
In order to overcome the drawbacks of the solutions cited here above, low profile polarizing surfaces operating with a single band in one polarization handedness for broadband satellite applications have been described in the two following documents.
As a first document, the article from K. Kärkkäinen et al., entitled “Frequency selective surface as a polarization transformer”, IEE Processing—Microwave Antennas Propagation, vol. 149, no. 516, pp. 248-252, 2002, describes doubly periodic planar metallo-dielectric arrays supported by a ground plane. When thermal losses or grating lobes are neglected, these structures fully reflect incident plane waves in a specular direction with a tailored phase shift. Among those surfaces, anisotropic designs impose a differential phase shift to the two polarizations of the incoming plane wave. A reflected circularly polarized wave can hence be achieved by means of the differential reflection phase provided by an anisotropic impedance surface.
As a second document, the article from E. Doumanis et al., entitled “Anisotropic Impedance Surfaces for Linear to Circular Polarization Conversion”, IEEE Trans. Antennas and Propagation, vol. 60, no. 1, January 2012, pp. 212-219, describes anisotropic impedance surfaces for linear to circular polarization conversion having a same structure as one of the first document.
According to the second document, circular polarization is characterized by electric field where the two orthogonal components are of the same amplitude and 90 degrees (or odd multiples of) out of phase. A linearly polarized wave may be converted to a circularly polarized wave by means of an engineered reflector, which provides this difference in phase between two crossed linear components. By virtue of anisotropy, it is possible to independently control or tune the reflection characteristics of two orthogonal linearly polarized incident plane waves and therefore achieve linear to circular polarization conversion.
The design consists in a regular array of rectangular patches above a ground plane and the phase response is tuned to reflect the two orthogonal plane waves defined with the electric field first x and second y axes (specular TE/TM Floquet modes) in quadrature over a wide frequency range. As a consequence, a linearly polarized plane wave with an inclination of 45 degrees with respect to the x and y axes of the structure would generate at normal incidence a purely circularly polarized signal with the same handedness over the full frequency range. The parameters to tune the response of the surface are the substrate parameter (dielectric constant εr and thickness h), the shape of the rectangular patch (a, b) and its periodicity (dx, dy).
As reported in the second document, such a design exhibits wide frequency band and stable performance in terms of axial ratio with the angle of incidence. This design is considered industrially relevant as it can reuse all the developments related to reflect array antennas for space applications. Having only one layer it is also very attractive as misalignment issues between layers are avoided, leading to better manufacturing yield. Typical results reported in the second document indicate an axial ratio better than 1 dB over wide frequency bandwidths but the concept is often restricted to narrow angular range.
These concepts reported in the same document have elongated profiles. The elementary cell consists of a dipole arranged in a rectangular lattice, very small along the x axis (around 0.1λg at central frequency, where λg refers to the guided wavelength), but large along the y axis (0.65λg at central frequency and up to 0.85λg at the highest frequency of the band). This feature makes the design stable to the angle of incidence in the x axis but liable to grating lobes in the y axis, even at very low angles of incidence.
Recently, the low profile polarizing surface of the second document was upgraded to dual band applications with orthogonal polarizations as described in a third document of N. J. G. Fonseca et al., entitled “High-Performance Electrically Thin Dual-Band Polarizing Reflective Surface for Broadband Satellite Applications”, IEEE Transactions on Antennas and Propagations, vol. 64, no. 2, February 2016, pp. 640-649. In this anisotropic impedance surface a same linear polarization is converted into a given circular polarization handedness over the first frequency band and into the orthogonal one over the second frequency band. This feature is of interest for communications satellite applications as most of the existing systems use orthogonal polarizations over transmit and receive frequency bands. In this design the longest unit cell dimension is 1.7λg at the higher operating frequency, and blinding effects can be clearly shown all over the band.
Besides, the polarizing surfaces described here above and reported so far in the state of the art have been designed and characterized only for a plane wave excitation. In addition, no polarizing reflectors with a curved profile, such as a paraboloid for example, have been reported. Since such polarizing reflectors span a wide range of angle of incidence, it is an objective to reduce the size of the cell for overcoming the sensitivity to the angle of incidence while maintaining the large band characteristics.
A first technical problem is to increase the stability and/or decrease the sensitivity of the axial ratio with the angle of incidence exhibited by high performance electrically thin polarizing surfaces for broadband satellite applications that convert a same linear polarization into a given circular polarization handedness over one frequency band, or into a given circular polarization handedness over a first frequency band and into the orthogonal one over a second frequency band.
A second technical problem, connected to the first technical problem, is to reduce the size of the elementary cell of such polarizing surface while maintaining the level of axial ratio sensitivity to the angle of incidence and the wide band or dual-band characteristics.